Unable to drop table, error mentions "reltriggers"

Hello,
I have two tables as follows:

create table vm_message
(
MessageId varchar(128) not null,
MessageType integer not null default 1,
UseCount integer not null default 1,
Sender varchar(256) not null,
Urgent boolean not null default
false,
Private boolean not null default
false,
MessageLength integer ,
MessageSize integer ,
MessageTime bigint ,
AttachedMsgId varchar(128) ,
MessageOid OID ,
UTCTime varchar(32) ,
EndTime bigint not null default
9223372036854775807,
ReceiveTime bigint not null default 0,
NdrReason integer ,
StartTime bigint default 0,
MWIOn boolean not null default
false,
TotalMessageLength integer not null default 0,
uid serial not null UNIQUE,
CallerPhoneNumber varchar(15) not null default '',
foreign key (AttachedMsgId) references vm_message on delete cascade
on update cascade,
primary key (MessageId)
);

create table vm_future_msg_job
(
MessageId varchar(128) not null,
Recipients varchar(1024) not null,
StartTime bigint not null,
JobType int default 0,
foreign key (MessageId) references vm_message on delete cascade on
update cascade,
primary key (MessageId,Recipients)
);

When I try to drop either one of these tables, I get the following
error:
ERROR: relation "vm_message" has reltriggers = 0
and the table does not get dropped.

What does this error mean???